for SOfIA Using Superconducting Hot-

Fabrication of a Terahertz Reciever Array
for SOfIA Using Superconducting Hot
-
Electron Bolometer (HEB) Mixers and
Micromachined Waveguide Components

Aaron Datesman

University of Virginia Microfabrication Laboratory

With credit to Art Lichtenberger and Jon Schultz (UVA), Chris Walker and
Dathon Golish (UAZ), and Jacob Kooi (CalTech)

OUTLINE:

•

Motivation: Submillimeter Radio Astronomy and SOfIA

•

Overview: Spectroscopy and Downconversion

•

A Small Amount about Mixers and Superconductivity

•

Design & Construction of the 1x5 Terahertz Array

•

Two Significant Fabrication Issues

•

HEB Physics and Operation

FABULOUS ACRONYMS!

•

SOfIA = Stratospheric Observatory for Infrared Astronomy

•

HEB = Hot
-
Electron Bolometer

•

FIB = Focused
-
Ion Beam

SOfIA’s Submm Science:

•
Interstellar cloud physics and star
formation in our galaxy.


•

Proto
-
planetary disks and planet
formation in nearby star systems.

•

Origin and evolution of biogenic
atoms, molecules, and solids.

•

Composition and structure of
planetary atmospheres and rings,
and comets.

•

Star formation, dynamics, and
chemical content of other galaxies.


•

The dynamic activity in the
center of the Milky Way.

•

Ultra
-
luminous IR Galaxies
(ULIRGS) as a key component of
the early universe.

•

5
–

300 microns
wavelength

•

9 first
-
light instruments

•

First light winter 2005!

www.sofia.arc.nasa.gov

Protostellar 4448
-
mm
Nisini 1999 / CFA

Spectroscopy
–

Astrochemistry: build and verify models of stellar
creation and evolution, radiative processes and absorption, etc.

Submillimeter wavelength regime: very rich in
rotational and vibrational molecular transitions

Heterodyne Downconversion

To detect a spectral line is easy. To distinguish it from all of the other radiation
a detector absorbs at the same time is hard!

By mixing the RF signal with a reference provided by the experimenter, the
“Local Oscillator”, the spectrum is downconverted and may be analyzed.

Mixing Elements: Superconducting and Otherwise

Gerecht, 1998

Why Superconductors?

•

Superconductors have an energy gap of order ~meV (gap
frequency for niobium is ~ 700 GHz).

•

Cryogenic operation reduces noise.

•

Mixing is a process which requires a non
-
linear I(V)
characteristic. Many of the characteristics of superconductive
devices, including the R(T) transition, are also very non
-
linear.

Why Hot
-
Electron Bolometers (HEBs)?

•

Better noise performance than Schottky diodes.

•

SIS junctions become lossy above the gap frequency.

•

Requires less LO power than either Schottky diodes or SIS
junctions.

1.45 THz Receiver:

•

Feedhorn Block

•

HEB Block

•

Backshort Block

1.45 THz Feedhorn Block Half

•

Fabricated by laser microchemical etching of silicon

Laser Micromachining of Silicon

•

Fast laser microchemical etching without reference
to crystal planes

HEB Mixer: Waveguide probe, chokes, CPW output line

•

Electromagnetic simulation using HFSS

HEB: A thin microbridge of Nb contacting Au on each end

Focused
-
Ion Beam:
The FIB Series 200 from FEI

Applications:

•

Cross
-
sectioning / TEM prep

•

Trimming and IC repair

•

Compositional profiling

•

Micro
-

and nano
-
fabrication

Specifications:

•

< 80 Angstroms spot size

•

CAD control, 4096 x 4096 pixels

•

Milling and deposition (SiO & Pt)

•

SIMS and end
-
point detection

•

Secondary electron imaging

HEB sits on a 0.75 micron thick silicon nitride membrane

•

Placing the backshort underneath the mixer allows us to create an array

Bulk Micromachined Pyramidal Backshorts

•

The actual backshort will be laser
-
micromachined into this pedestal.

Fabrication Issue #1

Fabrication Issue #2

How Does It Work?

•

Bolometer is a thermal device

•

Thin superconducting film
absorbs RF radiation above gap
frequency

•

Diffusion of heat to contacts
creates temperature profile

•

Portion of microbridge is
superconducting (no R),
portion is normal
–

“Hot Spot”

•

Power ~ V
2

creates beats

•

Temperature distribution
follows envelope at IF

•

IF bandwidth ~ a few GHz

•

w
3 dB

~ 1/L
2